The invention relates to an instrument set for connecting vertebral bodies, comprising a screw which can be screwed into a pedicle and which has a screw head and a shaft, a head piece designed for coupling to a system of rods, which can be connected polyaxially to the screw and which has a sleeve-shaped tulip having a first insertion opening for insertion of the screw head in an insertion direction, and a tool.
An instrument set of this type is known from the prior art and is used, for example, in spinal surgeries for stabilizing or correcting the spinal column. In such surgeries, screw shafts are first screwed into the pedicles of adjoining vertebral bodies to be connected to one another. The head of each screw can be connected to one head piece. A typically rigid connecting rod is then coupled to head pieces of the screws longitudinally such that the vertebrae are held in the desired position by the rod.
In such a surgical procedure, the coupling of the rigid connecting rod to the head pieces presents a challenge. It is known from the prior art that so-called polyaxial screws simplify this step of the surgical procedure substantially. A polyaxial screw consisting of a screw and a head piece connected to the screw enables the shaft of the screw to pivot in relation to the longitudinal axis of the head piece within a certain range. The head piece is also capable of rotating around its longitudinal axis with the shaft in any pivot position. Inserting the connecting rod into head pieces that can move relative to the shaft of the screw is much easier than inserting the connecting rod into head pieces that are rigidly connected to the shaft of the screw. Moreover, a head piece that is rigidly connected to the shaft requires that the screw be positioned in a precisely defined, predetermined angle in the pedicle in order to obtain the desired alignment of the vertebral bodies. In contrast, head pieces that are movable relative to the shaft allow for greater tolerance during insertion of the screw and for a readjustment of the position of the vertebral bodies before the rod system is locked.
With a rod system known from the prior art, the screw is typically screwed into the pedicle with the head piece already attached to the screw. In this method, however, the polyaxial connection between the head piece and the screw proves a hindrance. In an effort to solve this problem, a polyaxial screwdriver was disclosed in EP 2 208 473 A1. In DE 2012 219 630 A1, a rotatable driving profile located within the screw head can be deflected such that it is aligned at all times with the longitudinal axis of the head piece.
In the case of osteoporotic vertebral bodies, securely fixing the screw into the bone in such a way as to prevent the screws from breaking out presents a challenge. In addition, throughout the course of the surgery it is important to minimize the amount of pressure applied to the vertebral bodies. To solve this problem, DE 102 46 177 A1 discloses inserting a cannulated screw, and then injecting filler material into the screw; the filler material exits through windows arranged in the shaft of the screw and fuses with the bone. The head piece is then fixed onto the screw via a head that can be frictionally joined to the shaft.
To enable a quick and uncomplicated surgical procedure, it is desirable for an instrument set of the aforementioned type to comprise a minimal number of individual parts and for these parts to be easily joined to one another.
As is discussed in Eur. Spine J. 2010 January; 19 (1); 144-146, however, in a significant number of patients (5-8%) the stabilizing system must be removed after surgery either because the implant is faulty or because the patient responds negatively to the implant.
In light of these problems in the prior art, it is the object of invention to further develop an instrument set of the aforementioned type in such a way that the burden on the patient during insertion and removal of the screw and the head piece into and from a vertebral body is reduced; it is a further object to provide a method for inserting a screw and a head piece into a vertebral body, and a use of the instrument set to stabilize the spinal column.
According to the invention, this object is attained in that the head piece has a saddle inside the tulip, the saddle having a receiving region for receiving the screw head in a first end region that faces the first insertion opening, and having a coupling region that can be coupled to the tool in a second end region opposite the receiving region, and the saddle being movable between a released position, which can be reached by inserting the screw head, and a locked position, which can be reached by displacing the screw head out of the released position opposite the insertion direction, wherein in the locked position, the screw head that has been received in the receiving region cannot be released from the receiving region, and the tool, which is coupled to the coupling region, can be used to fix the saddle in a released position that enables the release of the screw head from the receiving region.
An instrument set according to the invention comprises the screw, the head piece and the tool, and thus a small number of individual parts. This facilitates a quick surgical procedure. The screw has a shaft, which expediently has a thread for screwing into a pedicle in a screwing-in direction. At the end opposite the screwing-in direction, the screw has a screw head, which can be connected to the head piece. The screw is screwed into the bone without the head piece, and the head piece is then attached to the screw that is fixed in the bone. The screwing-in of the screw thus is not hindered by the head piece.
The head piece has a sleeve-shaped tulip, inside which a saddle is located. The screw head can be inserted in an insertion direction through a first insertion opening in the tulip, into the interior of the tulip.
The saddle has a receiving region for receiving the screw head in a first end region that faces the insertion direction. The head piece can be coupled to the screw via the saddle. According to the invention, the screw head and the saddle are connected in such a way that the shaft of the screw can pivot in relation to the longitudinal axis of the head piece and the head piece can rotate around its longitudinal axis in any pivot position of the shaft. The unit comprising screw and head piece thus forms a polyaxial screw.
In a second end region opposite the receiving region, the saddle has a coupling region which enables it to be coupled to the tool. The tool can be advanced to the coupling region through a second insertion opening in the tulip, opposite the first insertion opening, and can be coupled to said coupling region.
The saddle is able to move in the tulip in the axial direction between a locked position and a released position. In the locked position, the screw head is permanently joined to the receiving region and the head piece is thereby fixedly coupled to the screw. In the released position, the screw head can be connected to the receiving region and disengaged therefrom.
To connect the screw to the receiving region, the saddle must be in the released position. This can be accomplished, for example, by inserting the screw head in the insertion direction through the first insertion opening and into the head piece. The screw head can then abut against the edge of the receiving opening, and the saddle can be pushed from its initial position into the released position by the movement of the screw in the direction of insertion. However, the screw head may also be moved into the released position by means of the tool, as will be described in further detail below. The saddle can then be fixed in the released position by means of the tool, which is coupled to the coupling region. The screw head can then be placed in the receiving region and coupled to the receiving region.
When the head piece and the screw are then moved apart, the saddle coupled to the screw head can be moved into the locked position.
To disengage the screw head from the receiving region, the saddle must be moved into the released position. This can be accomplished with the help of the tool, as will be described in greater detail below. The saddle can then be fixed in the released position by means of the tool, which is coupled to the coupling region. When the screw and the head piece are then moved apart, the tool continues to fix the saddle in the released position, and the screw head can be disengaged from the receiving region.
The connection of the invention between the screw and the head piece is capable of withstanding strong forces, which is absolutely essential in a polyaxial screw for stabilizing the spinal column.
The coupling and separation of the head piece and the screw that is fixed in the pedicle can be accomplished with the help of the tool in such a way that only minimal force is exerted on the vertebral bodies. This is advantageous particularly with osteoporotic bones.
An instrument set according to the invention enables a quick and simple disengagement of the head piece from the screw. And unlike many conventional systems, the screw can be left in the bone. This simplifies the replacement or removal of the head piece considerably and may also result in a shortening of the time required for the surgery.
A further advantageous embodiment of the invention is characterized in that the receiving region, which is delimited by a wall and has a receiving opening for insertion of the screw head, is reversibly deformable in a transverse direction that extends transversely to a longitudinal axis of the tulip. The wall of the receiving region and the receiving opening can then be expanded in the direction of the tulip wall and returned to their original shape. The reversible deformability of the receiving region allows its size to be adjusted, thereby enabling the screw head to be received in and disengaged from the receiving region. The wall and/or the receiving opening may be reversibly deformable.
The receiving region may be elastically deformable. In that case, the screw head can engage in the receiving region. However, the receiving region may also be plastically deformable.
The wall of the receiving region may be configured such that the receiving region is complementary to at least one region of the screw head. At least one portion of the wall of the receiving region and/or the receiving region opening may contact at least one region of the screw head which is arranged in the receiving region. This enables a movement of the screw to result in a movement of the saddle.
Also advantageous is an embodiment in which the wall of the receiving region is subdivided by at least two slots and/or windows into at least two separate sections. The slots and/or windows can enable deformability even if a hard material is chosen for the wall of the saddle. The same material can then be used for the saddle and the tulip. In that case it is advantageous for the slots/windows to breach the edge of the receiving opening with a component perpendicular to the receiving opening. This promotes the deformability or expandability of the receiving opening in a direction transversely to the direction of insertion. The wall may be subdivided by the slots/windows into two or more sections. As the number of separate sections increases, the deformability of the receiving region can be increased. This may, however, also decrease the stability of the connection between the screw head and the receiving region. An embodiment in which the wall is subdivided into four sections is advantageous.
In an advantageous embodiment of the invention, the slots and/or windows may be embodied as T-shaped. In this case it is advantageous for the vertical bar of the T to extend from the receiving opening of the receiving region substantially perpendicular to the receiving opening and to then merge with the horizontal bar of the T, which then extends substantially parallel to the receiving opening or in the circumferential direction of the saddle. With this configuration, the wall has a wall section, spaced at a distance from the receiving opening, the expansion of which parallel to the receiving opening is reduced to a thin material bridge by the horizontal bars of two adjacent T's. If the width of the material bridge is reduced, the deformability of the receiving region is increased or the (restoring) force that counteracts a deformation is decreased.
In a further possible embodiment of the instrument set, at least in the locked position, a wall of the tulip may limit the deformation of the receiving region in the transverse direction. This allows the screw head, in the locked position, to be inseparably connected to the receiving region. If the receiving region is plastically deformable, the wall, which limits the deformation of the receiving region in the locked position, can return the receiving region, which is expanded in the released position, to its original shape in the locked position. In the released position, the wall of the tulip may also limit the deformation of the receiving region in the transverse direction. However, it may also allow unhindered deformation of the receiving region in the released position.
In a further advantageous embodiment of the invention, a first head section of the screw head can have a maximum width BK in a direction perpendicular to the shaft, and a second head section located between the first head section and the shaft can have a second width bK in the direction perpendicular to the shaft, the second width being smaller than the maximum width BK. The screw head may be embodied as spherical, for example. However, it may also be embodied as polyhedral. Preferably, the screw head is in the form of a spherical shell. In that case, the receiving region is preferably complementary to a region of the spherical shell which extends at least from an end region of the screw head opposite the shaft up to the second head section. This allows the screw head to be rotated particularly uniformly in the receiving region. The second width bK may be smaller than the maximum width of the shaft. However, it may also be greater than the maximum width of the shaft.
In a practical embodiment of the invention, when the saddle is in the locked position, the wall of the tulip can limit the maximum expansion of the receiving opening that can be achieved by transverse deformation to a locked expansion (DS), which is smaller than the maximum width (BK) and greater than or equal to the second width (bK) of the screw head, and when the saddle is in the released position, the wall of the tulip can allow a maximum expansion of the receiving opening that can be achieved by transverse deformation to a released expansion (DF), which is equivalent to at least the maximum width (BK) of the screw head. When the saddle is in the locked position and the screw head has been inserted through the first insertion opening into the tulip, a section of the screw head that may coincide with the first head section abuts against the receiving opening in the receiving region. Since the maximum expansion of the receiving opening in the locked position is smaller than the maximum width (BK) of the first head section, when the screw is advanced further in the direction of insertion, the saddle is shifted in the direction of the released position. In that position, the wall of the tulip allows an expansion of the receiving opening to an expansion (DF), which is equivalent to at least the maximum width (BK) of the screw head. When the saddle is then fixed in the released position, for example by means of the tool, and the screw is moved further in the direction of insertion, the screw head exerts pressure on the receiving opening. In the released position, the receiving opening along with the wall of the receiving region can deform in the direction of the wall of the tulip, thereby expanding the receiving region. The receiving opening can expand to a released expansion (DF), which is equivalent to at least the maximum width (BK) of the screw head. This allows the screw head to be inserted into the receiving region. If the wall and the receiving opening of the receiving region are elastically reversibly deformable, then the receiving opening of the receiving region can be decreased again to its original size once the screw head has been inserted. The screw head is then positioned in the receiving region of the saddle, at least partially encompassed by the wall of the receiving region. One section of the screw head may contact the edge of the receiving opening. If the second width (bK) is equal to the expansion of the receiving opening, the second section of the screw head may touch the edge of the receiving opening. When the fixation of the saddle in the released position by means of the tool, which may be necessary, is then released and the screw is moved opposite the direction of insertion, a region of the screw head may come in contact with the receiving opening and the axial force may be transmitted to the saddle, moving the saddle from the released position into the locked position. In the locked position, the maximum expansion of the receiving opening is limited to the locked expansion (DS), which is smaller than the width (BK) of the first head section. As a result, the screw head cannot be disengaged from the receiving region by a movement of the screw and the head piece in the opposite direction.
In a further embodiment of the invention, an inner diameter (DImin) of the tulip at the first insertion opening may be minimal and an inner diameter (DImax) of the tulip in a plane defined by the receiving opening of the saddle in the released position may be maximal. In that case, the wall that delimits the inner region of the tulip can have a recess that encircles the wall in a closed contour in the region of the released position. The wall may, however, also have a plurality of recesses in the region of the released position that are separate from one another and allow an expansion of the receiving opening. The wall that delimits the inner region of the tulip may taper conically from the recess(es) to the first insertion opening.
Also advantageous is an embodiment in which the axial movement of the saddle in the tulip is limited between a first position and a second position. This prevents the saddle from falling out of the tulip. The first and second positions may be selected such that the saddle can move only entirely within the tulip.
According to the invention, the first position may coincide with the released position. The screw head can then be connected to the receiving region without the saddle having to be fixed in the released position by means of the tool. This is because in this embodiment, the screw head inserted into the first insertion opening of the tulip can push the saddle up to the released position. Since the saddle can be moved in the direction of insertion only up to the released position, the saddle is fixed in the released position by the movement of the screw in the direction of insertion, and the pressure exerted by the screw head on the receiving opening causes the receiving opening to expand, allowing the screw head to be inserted into the receiving region.
Also advantageous is an embodiment in which the saddle has at least one projection between the receiving region and the coupling region, which projects up to the wall and engages into a recess formed in the wall of the tulip, wherein the projection, which abuts against first and second stop regions that delimit the recess in the axial direction when the saddle is moved axially, can limit the movement of the saddle between the first and second positions. In this case, the wall may have a single projection. However it may also have a plurality of projections. Preferably, the wall has two projections diametrically opposite one another. The projection may be cuboid in shape. For each projection, a recess is provided in the wall. The recess is embodied such that each projection can engage in its respective recess. Each recess has a first and a second stop region, each region having a component perpendicular to the insertion opening and limiting the expansion of the recess in the axial direction. When the saddle is moved in the direction of insertion, a first stop surface of the projection having a component perpendicular to the direction of insertion abuts against the first stop region, thereby limiting the movement of the saddle in the direction of insertion. This defines the first position. When the saddle is moved opposite the direction of insertion, a second stop surface of the projection having a component perpendicular to the direction of insertion abuts against the second stop region, thereby limiting the movement of the saddle opposite the direction of insertion. This defines the second position. The first and second stop surfaces of the projection are preferably complementary to the surfaces of the first and second stop regions, respectively. As a result, the contact surface of the stop surface that abuts against a stop region is maximal.
The positioning of the projection and the recess may also be interchanged. For instance, the wall may have one or more projections, each of which can engage in a complementary recess in the saddle, thereby limiting the movement of the saddle in the tulip.
In an advantageous embodiment of the invention, the tulip and the saddle may be detachably connected to one another. In that case, the tulip and the saddle can be produced on different production lines and connected to one another either by the manufacturer or by the consumer.
In a further embodiment of the invention, the tulip can have a second insertion opening in a second end region opposite the first insertion opening, said second insertion opening having an edge region in which at least one, preferably two diametrically opposite indentations for receiving a rod of the rod system are formed. The rod can then be inserted into the indentation(s) starting from the second insertion opening. The edge of each indentation may be embodied as U-shaped. In that case, the arms of the U can extend from the insertion opening substantially parallel with the longitudinal axis of the tulip in the direction of the first insertion opening, and can then be connected to one another by the curved portion of the U.
In an instrument set according to the invention, the saddle can have a recess for receiving the rod located in the indentations of the edge region of the tulip, with said recess extending diametrically through the saddle in a second end region located in the direction of the second insertion opening of the tulip. The recess may be embodied as U-shaped. In that case, the arms of the U can extend from the second end region of the saddle substantially parallel to the direction of insertion up to the receiving region, and can then merge into the curved portion of the U before reaching the receiving region. In that case, the curvature of the curved portion of the U and the spacing between the arms are embodied such that the mobility of a cylindrical rod received in the recess in a direction perpendicular to the axis of the rod and perpendicular to an axis of the head piece is minimal.
According to the invention, the tulip may have a locking device located in the second end region for the purpose of fixing the rod. With the locking device, the rod can be secured to the head pieces of the screws that have been screwed into the pedicle, such that the corresponding vertebrae can be fixed in the desired position. The locking device may have an internal thread located in the second end region of the tulip. The internal thread may be screwable to a set screw. This can enable the desired locking of the rod in the head piece.
According to the invention, the tool may comprise a holding grip having a hollow interior and having a coupling member for coupling to the coupling region of the saddle, in which case the coupling member may be at least partially located within the interior space and rotatable about a longitudinal axis (A). The coupling member may be embodied as cylindrical. In addition, the coupling member may have a twist grip in a second end region opposite the first end region. The tool can then be held and guided using the holding grip. The holding grip may have grooves and/or windows that are designed to improve the grip on the holding grip. The coupling member can be rotated around its longitudinal axis using the twist grip. The twist grip may have grooves and/or edges that improve the grip on the twist grip.
In a further embodiment of the invention, the saddle can be non-rotatably arranged in the tulip. According to the invention, the coupling region of the saddle may have an internal thread. Additionally according to the invention, the coupling member of the tool may have an external thread in a first end region, designed to be screwed together with the internal thread of the coupling region. The tool may further have a locking device with which the holding grip can be connected in a rotationally fixed manner to the tulip. The locking device may have two projections that project outward from the holding grip and can be brought into engagement with the recesses of the tulip. The projections may be arranged diametrically opposite one another. In addition, each projection can have a U-shaped contour. In that case, the arms of the U expediently extend from the first edge region of the tool substantially parallel with the longitudinal axis of the tool, and are then connected to one another via the curved portion of the U. This embodiment is particularly advantageous if the end region of the tulip located at the second insertion opening has two U-shaped indentations arranged opposite one another. The U-shaped projections are then expediently embodied as complementary to the U-shaped recesses.
When the external thread of the coupling member of the tool is then screwed to the internal thread of the coupling region of the saddle, since the saddle is arranged rotationally fixed in the tulip and since the holding grip of the tool is coupled to the tulip in a rotationally fixed manner, the rotational movement of the coupling member is converted to an axial movement of the saddle. The saddle can thus be moved from the locked position to the released position. The saddle can also be fixed in the released position by its coupling to the coupling member of the tool.
In a further embodiment of the invention, the coupling member may be displaceable in relation to the holding grip in the axial direction between a first end position and a second end position. In the first end position, the coupling member is retracted into the interior of the holding grip. In this case, the coupling member can be retracted into the interior of the tool far enough that the first end region with the external thread is positioned entirely within the interior space. In the second end position, the coupling member can protrude in the axial direction beyond a first holding grip end region of the holding grip located in the direction of the coupling member. The coupling member can be displaced, for example, by holding the tool by the holding grip and holding the coupling member by the twist grip and then moving the coupling member in the axial direction. This allows the screwing of the coupling member into the coupling region to be simplified as follows: The tool can be connected in a rotationally fixed manner to the tulip using the coupling member located in the first end position via the locking device of the tool. When the first end region of the coupling member is located entirely within the interior space of the tool, the tool can be connected particularly easily to the tulip. The coupling member can then be pushed toward the second end position and thus toward the tulip until the external thread of the coupling member can be brought into engagement with the internal thread of the coupling region of the saddle. By turning the twist grip, the coupling member can then be screwed into the coupling region. As described above, the rotational movement of the coupling member can be converted into an axial movement of the saddle, and the saddle can be moved into the released position.
In an instrument set according to the invention, the screw may be solid or cannulated and/or fenestrated. A cannulated screw can be screwed into the bone with the help of a guide wire. The guide wire can facilitate positioning of the screw. In the case of a cannulated and fenestrated screw, a fill material may also be injected into the cannula, exiting through the windows and fusing with the bone. This can facilitate fixation in osteoporotic bone.
In an instrument set according to the present invention, screws having a screw length of 30 mm to 100 mm are preferably used. Screws having a shaft diameter of 4.5 mm or 5.5 mm or 6.5 mm or 7.5 mm or 8.5 mm or 9.5 mm are preferably used.
According to the invention, the screw may have a first thread with a first thread pitch and a second thread with a second thread pitch, and the first and second thread pitches may be the same or different. The use of different thread pitches can allow traction or pressure to be exerted on a bone or can even allow splintered bone pieces to be joined together.
The screw may also be embodied such that the first thread pitch and the second thread pitch are the same and the two threads are arranged along the shaft of the screw in such a way that the screw is double-threaded in a second shaft region adjacent to the screw head and is single-threaded in a first shaft region extending away from the second shaft region opposite the screw head. The anchoring of the screw in the bone can thereby be improved.
In a further embodiment of the invention, the screw head can have a recess for receiving a screwdriver. The recess may be embodied as a square socket, a hexagonal socket (Allen), a hexalobular socket (Torx) or a serrated socket (XZN).
In this description, the head piece is described in conjunction with the screw and the tool. However, protection is also being sought for the head piece alone, without the screw and the tool.
In this description, the head piece and the screw are described in conjunction with the tool. However, protection is also being sought for the head piece and the screw, without the tool.
In this description, the tool is described in conjunction with the head piece and the screw. However, protection is also being sought for the tool alone, without the head piece and the screw.
The invention further provides a method for inserting and for removing a screw according to the invention and a head piece according to the invention into and from a vertebral body, for example as part of a spinal surgery. In this method, first a guide wire can be emplaced. The guide wire can facilitate the setting of a cannulated screw in the correct position in the vertebral body. The screw may, however, also be screwed into the vertebral body without the guide wire. A screwdriver can be coupled to the recess of the screw head and the screw can be screwed into the pedicle. If a cannulated and fenestrated screw is used, the fixation of the screw in the bone can be improved by injecting a suitable material into the cannula of the screw, which then exits through the windows of the screw and fuses with the bone, particularly with an osteoporotic bone. The tool can then be coupled in a rotationally fixed manner to the tulip by means of the locking device. Locking may be accomplished, for example, by bringing two projections that are arranged diametrically opposite one another in the first holding grip region into engagement with two U-shaped indentations that are arranged diametrically opposite one another and formed in the edge region of the second insertion opening. The coupling member can thereby be held in the first position. The tool can be held by its holding grip. The coupling member can be pushed through the second insertion opening in the tulip toward the second end position, until the coupling member abuts against the coupling region of the saddle. The twist grip of the coupling member can then be rotated, allowing the external thread of the coupling member to be screwed into the internal thread of the saddle. The rotational movement of the coupling member can thereby be converted into an axial movement of the saddle, as described above. The saddle can thus be moved into the released position. The saddle can be fixed in the released position by means of the tool. The head piece can then be moved with the first insertion opening up to the screw head of the screw that has been screwed into the pedicle. With this movement, the screw head can be guided in the direction of the saddle. Since the saddle is fixed in the released position, the screw head can be inserted without pressure into the receiving region.
The coupling member can then be unscrewed from the internal thread of the saddle by rotating the twist grip. This allows the saddle to be moved into the locked position. This method of insertion exerts only a small amount of pressure on the vertebrae and is particularly advantageous in the case of osteoporotic bone. The tulip is then connected polyaxially to the screw. The head piece may, however, also be mounted on the screw head without the tool. To accomplish this, the head piece can be introduced into the screw in such a way that the screw head enters into the first insertion opening in the tulip. The head piece can be advanced further toward the screw. This allows the head of the screw to abut against the receiving opening of the saddle. The saddle can thereby be shifted from the locked position into the released position. If the released position coincides with the first position, the saddle cannot be moved any further in the direction of insertion. If the head piece is moved further toward the screw, the screw head will exert pressure on the receiving opening. This may cause the receiving region to expand in the radial direction. The screw head is then able to pass through the widened receiving opening into the receiving region. If the deformation of the receiving region is elastic, the receiving opening can then return to its original size. The screw head is snapped into the receiving region, and the screw is securely joined to the head piece.
In contrast to conventional methods, the method according to the invention for inserting the screw according to the invention and the screw head according to the invention does not entail the problem of a head piece which is polyaxially connected to the screw hindering the screwing of the screw into the pedicle.
The head piece can be easily disengaged from the screw by means of the tool. To accomplish this, the tool can be held by the holding grip and the coupling member can be moved to the first end position. In the first end position, the first end region of the coupling member having the external thread may be located entirely within the interior space of the holding grip. The tool can then be coupled with the locking device to the tulip in a rotationally fixed manner. Locking can be accomplished, for example, by bringing two projections that are arranged diametrically opposite one another in the first holding grip region into engagement with two U-shaped indentations that are arranged diametrically opposite one another and are formed in the edge region of the second insertion opening. The first end region of the coupling member having the external thread can then be pushed through the second insertion opening into the head piece and up to the coupling region of the saddle. The coupling member can then be rotated by means of the twist grip, thereby screwing the coupling member into the coupling region of the saddle. This rotational movement allows the saddle to be moved axially into the released position. The head piece can then be disengaged from the screw head with little applied force by moving the tool with the saddle coupled thereto away from the screw.
This method allows the head piece to be detached from the screw easily, quickly and with little force applied. This enables the head piece to be removed from the body easily without removing the screw from the bone.
An instrument set according to the invention can be used for stabilizing the spinal column. In such a process, first screws according to the invention can be screwed according to the method described above into each of the two pedicles of the vertebral bodies to be connected. A head piece according to the invention can then be attached to each of the screws, as described above. The head pieces can be attached with or without the help of the tool. One rod can then be joined to each of the head pieces arranged lengthwise along the spinal column. Since the head pieces are connected polyaxially to the screws, the rods can be inserted more easily than if the head pieces were rigidly connected to the screws. The polyaxial connection between the head piece and the screw then allows the positioning of the vertebral bodies to be readjusted. Finally, the two rods can be locked in the predetermined position by means of the locking device. The vertebral bodies are then fixed in a predetermined position.
The screw, the tulip and the saddle may be made of the same material, for example Ti-6AI-4V. They may, however, also be made of different materials. The material may include a stainless steel, for example, Co-28Cr-6Mo or X2CrNiMo18-15-3 (1.4441). It may also be made of a different material.